It takes a battery lamp w/ a square metal housing holding two D-cell batteries which mounts on the bracket, and round lamp w/ reflector on the front of the battery housing. The battery housing has a U-shaped bracket on the back which slides down over the headset mounting bracket. It looked similar to the headlamp in the attached photo, except the mounting bracket location is different...

I'm pretty sure they still sold those lights in the US through the end of the 60's at least; by 1970 the 10-speed had taken over from the 3-speed English racer as the adult bike of choice in the US, but they still may have made those lights for the UK market, those Brits are pretty traditional...